The underside of an overhanging cornice of a building extending out from the plane of the building walls.
Merriam-Webster Online Dictionarysoffit (noun)
the underside of a part or member of a building (as of an overhang or staircase) , especially the intrados of an arch
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A soffit is an exterior or interior architectural feature, generally the underside of any construction element. A structure to fill the space between the ceiling and the top of cabinets mounted on the wall is called a soffit, as is the material connecting an exterior wall to the edge of the roof under the eaves.